The invention relates to a process for the fractional condensation of an acrylic acid-containing product gas mixture of a gas-phase partial oxidation of C3 precursors of acrylic acid with molecular oxygen under heterogeneous catalysis precursors of acrylic acid with molecular oxygen under heterogeneous catalysis in a separation column containing hydraulically sealed cross-flow trays as baffles having separation activity.
Acrylic acid is an important intermediate which is used, for example, in the preparation of polymer dispersions.
Acrylic acid is obtainable, inter alia, by gas-phase partial oxidation of C3 precursors of acrylic acid with molecular oxygen under heterogeneous catalysis over solid catalysts at elevated temperatures. The term xe2x80x9cC3 precursorsxe2x80x9d of acrylic acid covers those chemical compounds which are formally obtainable by reduction of acrylic acid. Known C3 precursors of acrylic acid are, for example, propane, propene and acrolein. However, compounds such as propionaldehyde or propionic acid are also among the C3 precursors. Starting from them, the gas-phase partial oxidation with molecular oxygen under heterogeneous catalysis is at least partly an oxidative dehydrogenation.
In the novel gas-phase partial oxidation under heterogeneous catalysis, said C3 precursors of acrylic acid are as a rule diluted with inert gases, such as nitrogen, CO, CO2 and/or steam, are passed, in the form of a mixture with molecular oxygen, at elevated temperatures and, if required, superatmospheric pressure, over transition metal mixed oxide catalysts and converted by oxidation into an acrylic acid-containing product gas mixture.
DE-A 19740252, DE-A 19740253, DE-A 19833049, DE-A 19814375, DE-A 19814421 and DE-A 19814449 disclose that the basic separation of the acrylic acid contained in the product gas mixture from acrylic acid-containing product gas mixtures of gas-phase partial oxidations of C3 precursors of acrylic acid under heterogeneous catalysis is possible by subjecting the product gas mixture, if necessary after direct and/or indirect prior cooling, to a fractional condensation while sending into itself in a separation column provided with baffles having separation activity, and removing the acrylic acid as crude acrylic acid via a side take-off of the separation column. The term crude acrylic acid expresses the fact that the acrylic acid removed via the side take-off is not a pure product but a mixture which also contains typical byproducts of the gas-phase oxidation (e.g. water, lower aldehydes, acetic acid, propionic acid, etc.) in addition to acrylic acid (as a rule xe2x89xa795% of the weight of the mixture).
Compared with the other known processes for the basic separation of a crude acrylic acid from the product gas mixtures of gas-phase partial oxidations of C3 precursors of acrylic acid under heterogeneous catalysis, which are usually carried out by taking up the acrylic acid in a suitable absorbent and then removing the absorbent by distillative separation methods, the typical feature of the fractional condensation method outlined above is that undesired polymer formation occurs to a lesser extent on addition of polymerization inhibitors.
The abovementioned prior art publications recommend in particular stacked packings, dumped packings and/or trays, preferably bubble trays, sieve trays, valve trays and/or dual-flow trays, as baffles having separation activity in the separation columns to be used for the fractional condensation of the product gas mixture of the gas-phase partial oxidation of C3 precursors of acrylic acid under heterogeneous catalysis. In the exemplary embodiments, the only separation columns used are those which contain either only bubble trays (hydraulically sealed cross-flow trays) or only dual-flow trays as baffles having separation activity. The disadvantage of the prior art recommendations is that stacked packings and dumped packings, on the one hand, promote polymer formation and, on the other hand, rapidly lose their permeability when undesired polymer formation occurs. Another disadvantage, when dual-flow trays are used alone as baffles having separation activity, is that the separation efficiency of the column is not completely satisfactory. When bubble trays are used alone, the separation column loses its permeability as a result of polymer formation even after operating times which are not completely satisfactory.
It is an object of the present invention to provide a process for the fractional condensation of an acrylic acid-containing product gas mixture of a gas-phase partial oxidation of C3 precursors of acrylic acid with molecular oxygen under heterogeneous catalysis in a separation column containing hydraulically sealed cross-flow trays as baffles having separation activity, which process reduces the disadvantages of said prior art processes.
We have found that this object is achieved by a process for the fractional condensation of an acrylic acid-containing product gas mixture of a gas-phase partial oxidation of C3 precursors of acrylic acid with molecular oxygen under heterogeneous catalysis in a separation column containing hydraulically sealed cross-flow trays as baffles having separation activity, wherein the separation column used is one which contains, from bottom to top, first dual-flow trays and then hydraulically sealed cross-flow trays as baffles having separation activity.